Introduction to Oracle Compression features

Everyone hears this word, must think of save hard disk space. Yes, it is used to do this, but to extend it, it can not only save hard disk space, but also can save bandwidth and memory use (CPU has a little bit of loss because of the need for more computed value operations, but in most environments, the CPU is idle, so it can be ignored).

Basiccompression This feature has been available since 9i, and 11GR1 is starting to offer a new compression feature, the OLTP compression. Basic Compression is included in the EE version and does not require additional charges, but OLTP Compression requires additional Oracle Advanced Compression optionlicense. Senior? Want to use? Pay for it.

The following image will help you understand the principle of the compressed block:

In simple terms, there is a special data structure in the block, symbol table.

The contents of the symboltable dump are shown below:

tab 0, row 0, @0x1da4    
tl:7 fb:--h-fl--lb:0x0 cc:12 Col 0:*null* Col  1: [5]  4c 49 44    
Col  2: [1]  4e    
col  3:*null*    
Col  4: [1]  4e    
col  5: [1]  4e    
Col  6: [3]    
col  7: [7]  6f 0a 1e 0d    
Col  8:[19]  32 30 31 31 2  2d 3a 3132 3a    
Col 3a  9: [2]  C1    
col: [5] 4e    
col 11: [D 7] 6f 0a 1e 0d    
bindmp:00 2a 0c 15 20 24 22

The real data content looks like this:

tab 0, row 0, @0x1da4    
tl:7 fb:--h-fl--lb:0x0 cc:12 Col 0:*null* Col  1: [5]  4c 49 44    
Col  2: [1]  4e    
col  3:*null*    
Col  4: [1]  4e    
col  5: [1]  4e    
Col  6: [3]    
col  7: [7]  6f 0a 1e 0d    
Col  8:[19]  32 30 31 31 2  2d 3a 3132 3a    
Col 3a  9: [2]  C1    
col: [5] 4e    
col 11: [D 7] 6f 0a 1e 0d    
bindmp:00 2a 0c 15 20 24 22

A bit similar to the clustertable block structure. Simply put the block duplicates into a symbol table, and the real table row piece references the duplicate content stored in the symbol table. Because Basiccompression and oltpcompresssion are both in-block compression, even if they have duplicate content, they cannot achieve the compression effect if they are not stored in the same block. The principle of the first introduction to this, if you want to further study, you can refer to the old Bear's blog.

Oracle compressed table data block format resolution:

Http://www.laoxiong.net/dissect_compressed_block_part1.html